Capillaries are very thin walled in order to easily allow the exchange of gases. Gaseous exchange between the blood and tissues occurs at the level of the capillaries.

Arterioles are small branches of arteries with only one or two layers of smooth muscle in the tunica media. Arterioles regulate the amount of blood going into the capillary bed.

Venules are small branches of veins.

Elastic arteries are the arteries leaving the heart and the major branches. The aorta is an elastic artery.

Most of the named arteries are muscular arteries (with the exception of the aorta and the major branches off the aorta). The dividing line between elastic arteries and muscular arteries is not clear cut. However, a pronounced internal elastic membrane and external elastic membrane are distinguishing characteristics of muscular arteries.

Histology hint from Sarah Bellham: Elastic arteries also have an internal elastic membrane. However, there is so much elastic material in the tunica intima of an elastic artery, that a single, discrete internal elastic membrane is not visible.

The phenomenon when a vein is between two capillary beds is called a venous portal system. An example of this is the hepatic portal system. Another example of a venous portal system is seen in the brain between the hypothalamus and pituitary.

The phenomenon when an arteriole is between two capillary beds is called an arterial portal system. This is seen in the kidney.